Fiddleheads are the young coiled fern leaves (about an inch in diameter) of the ostrich fern (Matteuccia struthiopteris). Nearly all ferns have fiddleheads, but those of the ostrich fern are unlike any other. The storage life is around 10 days. You must cook fiddleheads, do not eat them raw or undercooked, as you can become ill. Fiddleheads are a good source of potassium; they also contain vitamin C, niacin and iron.